In recent years there has developed an increased concern regarding the transfer of disease, infection or the like to syringe users and healthcare professionals who accidentally or through negligent handling stick themselves with hypodermic needles while disposing of used hypodermic needle containing products. In many areas in a hospital, where needle cannula products are used, disposal bins are provided so that a syringe or other needle cannula product may be immediately discarded in a safe rigid container. However, there are areas of medical practice, such as emergency rooms, where disposal containers may not be readily available or practical, and where products having self-contained safety features are desirable. In theory, after such a syringe is used to inject medication or for another purpose, a safety device contained within the syringe or needle assembly is activated to prevent further contact with the sharp needle tip. One type of safety syringe includes structure which allows the withdrawal of the hypodermic needle into the syringe barrel to minimize the chance of further contact with the sharp needle tip.
One such prior art retractable needle syringe includes a frangible zone which allows the separation of the forward wall of the barrel, which is connected to the hypodermic needle, from the sidewall of the barrel. The syringe also contains structure on the interior of the forward wall and the exterior of the piston for selectively attaching the piston to the forward wall so that the user can forcibly twist the piston to break the frangible structure and draw the forward wall, including the hypodermic needle, into the syringe barrel. This design requires a compromise in the design of the syringe barrel. The barrel must be strong enough to remain intact during normal use yet weak enough to be sheared apart by any user regardless of strength.
The prior art also includes retractable needle syringes. These syringes have structure that engages a needle carrier allowing the needle carrier to be forcibly disengaged from the syringe barrel, by action of the plunger rod, and withdrawn into the syringe barrel. Many prior art retractable needle syringes have deficiencies similar to that described above. In particular, the needle or the needle carrier of the retractable needle syringe must be securely held by the syringe barrel during normal use which often includes substantial hydraulic pressures experienced during injection especially with highly viscous liquids, and forces including piercing rubber stoppers with medication vials. The syringe barrel must hold the needle carrier to a degree that it will not be overcome by the forces of normal use and will still be disengageable through forces applied to a plunger rod which extends from the open proximal end of the syringe barrel. Many prior art retractable needle syringe designs when made with sufficient strength to withstand the forces of normal use have a needle carrier which cannot be easily disengaged. On the other hand, easy disengagement of the needle or the needle carrier can lead to a structure which may not withstand the forces of normal use. This is especially true with needle carriers which are structured to allow a needle assembly to be installed and removed so that the user can select the hypodermic needle size at the time of use. These syringes must also resist the high torque and forces of needle installation and removal. In addition, retractable needle syringes require a two-handed withdrawal procedure which increases the difficulty of use.
The prior art also includes retracting needle syringes which include a spring loaded needle assembly which is held in position during normal use of the syringe assembly and a hollow plunger rod which is sealed during normal use of the syringe assembly so that medication may not enter the plunger rod cavity. These syringes must have structure to allow release of the spring loaded needle and the opening of the plunger rod cavity so that the needle may enter the plunger rod cavity after the syringe is used for its intended purpose. The retracting needle syringes have similar design problems as those recited hereinabove for retractable needle syringes. In particular, the cavity in the plunger rod must be sealed so that medication cannot enter the plunger rod during use. This seal must sometimes withstand high hydraulic pressures when injecting relatively viscous medication through small needles and still be capable of being easily unsealed and to allow access by the needle assembly. Likewise, the needle assembly must be firmly held in place through the forces of injection and still be disengageable so that it may retract into the syringe barrel and into the plunger rod. Some of the prior art retracting needle syringes use plugs to cover the plunger rod cavity leading to an arguably difficult situation since the plug may fail during the injection process. Likewise, some use plugs to hold the needle assembly in place which can arguably become dislodged during use causing fear of the syringe. Further, these designs do not allow for a replaceable needle assembly thus depriving the healthcare worker of the option of choosing the appropriate needle size for the injection or procedure being performed.
Although the prior art teaches many different retractable needle syringes and retracting needle syringes have the capacity to withdraw or allow the needle to enter the syringe barrel or the plunger rod there is still a need for a simple, straight-forward, reliable, easily fabricated retracting needle syringe having a well-sealed plunger rod cavity which can easily be unsealed at the time of needle retracting. There is also a need for a retracting needle syringe having adequate structural integrity to withstand the forces of injection and while the spring can still be easily and intentionally released to allow the needle assembly to enter the plunger rod cavity. There is also a need for a retracting needle syringe having replaceable spring-loaded needle assemblies to allow selecting the proper needle size at the time of use and to facilitate prefilling.